In gas turbine engines, for example, aircraft engines, air is drawn into the front of the engine, compressed by a shaft-mounted rotary compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on a shaft. The flow of gas turns the turbine, which turns the shaft and drives the compressor. The hot exhaust gases flow from the back of the engine, providing thrust that propels the aircraft forward.
Gas turbine engines generally include a high pressure compressor, a combustor, and a high pressure turbine. The high pressure compressor, combustor, and high pressure turbine are sometimes collectively referred to as a core engine. Such gas turbine engines also may include a low pressure compressor for supplying compressed air, for further compression, to the high pressure compressor, and a fan for supplying air to the low pressure compressor.
The high pressure compressor typically includes a rotor surrounded by a casing. The casing is typically fabricated to be removable, such as by forming the casing into two halves that are then removably joined together. The high pressure compressor includes a plurality of stages and each stage includes a row of rotor blades and a row of stator vanes. The casing supports the stator vanes, and the rotor supports the rotor blades. The rotor blade rows are adjacent stator vane rows, wherein each stage includes a set of stator vanes that direct air flow toward a downstream set of rotor blades.
To improve the overall operation of the compressor, several compressor stator vanes are rotatively mounted to allow each vane to rotate around its longitudinal axis (which extends in a radial direction from the centerline of the engine) to adjust the angular orientation of the vane relative to the airflow through the compressor. These variable stator vane assemblies are utilized to control the amount of air flowing through the compressor to optimize performance of the compressor. Each set of variable stator vanes includes an adjacent set of downstream rotor blades. The orientation of the variable stator vane affects air flow through the compressor. A lever arm is fixedly joined to the vane stem extending outwardly from the vane bushing. The distal end of the lever arm is operatively joined to an actuation ring that controls the orientation of the vane. All of the vane lever arms in a single row may be joined to a common actuation ring for ensuring that all of the variable vanes are simultaneously positioned relative to the airflow in the compressor stage at the same angular orientation.
A known variable vane assembly includes a variable vane, and a trunnion seal which may include a bushing and/or a washer. The variable vane assembly is bolted onto a high pressure compressor stator casing and the bushing and washer surround an opening that extends through the casing. The variable vane includes a vane stem that extends through the opening in the casing and through the bushing and washer. The bushing and washer are referred to herein as a bearing assembly. The bearing assembly produces a low friction surface that prevents metal on metal contact between the vane stem and the casing. Such variable vane assemblies have possible air leakage pathways through the openings in the casing. Also, the high velocity and high temperature air causes oxidation and erosion of the bearing assembly, which may accelerate deterioration of the bearing assembly, lead to failure of the bearing assembly, and eventual failure of the variable vane assembly.
Once the bearing assembly fails, an increase in leakage through the opening occurs, which results in a performance loss for the compressor. In addition, failure of the bearing assembly may result in contact between the stator vane and the casing, which causes wear and increases overhaul costs of the engine.
During operation, a gas turbine engine experiences a variety of forces within the engine that affect the bearing structures. For example, during a stall condition, forces on the vane assembly go through a reversal of direction, locally bending the case material that supports the bearing assembly. Such localized bending may result in strain and potential breakage of bearing components, particularly the bushing. High temperature or ceramic bearing materials have an elastic modulus that is much greater than the materials within the vane assembly. The result of the bearing assembly having a much greater elastic modulus is that the bushing and washer are less able to elastically deform with the case, due to the relative stiffness of the bushing/washer material. Therefore, the bushing and washer bearing structures are more susceptible to breakage when exposed to forces, such as the forces experienced during a stall condition.
A number of structures in the gas turbine engine, including the bushing and washer structures, used with variable stator vanes are subjected to conditions of wear at temperatures ranging from low temperatures to highly elevated temperatures. In addition, the bushing and washers are subject to high altitude atmospheres. In addition to low temperatures, high altitude atmosphere includes little or no water vapor.
One known material for fabrication of bushings for variable stator vane assemblies is a specially developed composite of carbon fiber reinforcing materials in a polyimide resin matrix manufactured by E. I. Du Pont De Nemours and Company of Wilmington, Del. The bushings are commonly known as VESPEL®CP™ bushings. VESPEL® and CP™ are trademarks that are owned by E. I. Du Pont De Nemours and Company. The polyimide resin used in the VESPEL®CP™ bushings is commonly known as NR150™. The NR150™ trademark is owned by Cytec Technology Group of Wilmington, Del. Although the VESPEL®CP™ bushings have an extended life at temperatures 450–500° F. (232–260° C.), the VESPEL®CP™ bushing have an upper temperature limit of 600° F. (316° C.). Extended operation at temperatures at or above 600° F. (316° C.) limit their operational life. The polymer matrix bushings do not withstand the combinations of high temperature and vibrational loading experienced in the operation of the gas turbine engine well, leading to a relatively short part life.
Another known method for reducing wear on the variable stator vane assembly is placing a carbon-containing antifriction coating on a surface in the variable stator vane assembly. This antifriction coating is fabricated from a material that reduces the coefficient of friction between the surface of the trunnion and the surface of the casing. One carbon-containing component known for lubricant coating is graphite. However, graphite has the disadvantage that water vapor is required to maintain lubricity. Atmospheres at aircraft cruise altitudes do not have enough water vapor present for graphite to be lubricious. Graphite also has the disadvantage of poor tribological properties in applications that require reciprocating motion. An additional disadvantage of graphite is that graphite begins to oxidize rapidly at temperatures around 500° C. (932° F.) and greater. Some variable stator vane systems may experience temperatures in excess of 500° C. (932° F.). Therefore, a replacement material for graphite in antifriction coating is needed.
Attempts have also been made to coat the stator vane trunnion with a wear coatings. The wear coatings previously attempted to incorporate known lubricant compositions (e.g., graphite) with low coefficient of friction with a hard, smooth wear coating having a wear resistant coating material on the vane trunnion. However, the materials used for known wear coatings lack the ability to maintain the properties of each of the individual components (i.e., fails to maintain both low coefficient of friction and wear resistance) and reduces the fatigue strength of the coated material. In other words, the wear coatings do not provide all of the desired tribological properties (e.g., reduced wear and low coefficient of friction) and mechanical properties (e.g., fatigue strength) required for extended operation of variable stator vanes subject to conditions of high temperature, vibration and high altitude atmospheres.
Accordingly, it would be desirable to provide bearing assemblies fabricated from materials having performance characteristics that will reduce or eliminate air leakage between the stator vane stem and the compressor casing while providing an increase in the wear resistance and durability of the bushing and washer to increase part life in high temperature and vibration loading applications. In addition, it would be desirable to provide coating systems that provide the desirable tribological properties and the desirable mechanical properties in order to resist wear and provide operation in a variety of atmospheres. The present invention fulfills this need, and further provides related advantages.